[lmeyerov]

Summary

• Preserve nonempty flat-chain wire bodies so remote Cypher/GFQL callers retain an explicit query payload.
• Send Let/DAG programs through gfql_query and stop substituting an empty legacy operations payload.
• Reject unsupported or malformed remote query payloads with structured errors instead of treating an identity chain as a compatibility sentinel.
• (Now in ci(spark): pin Temurin 17 for PySpark tests #2245) The Spark CI JVM pin to Temurin 17 landed via ci(spark): pin Temurin 17 for PySpark tests #2245; this PR contains no workflow changes.

Tests

• Focused wire-path and structured-error regression tests for chains, Let/DAG payloads, and rejected query types.
• Exact-head CI, CodeQL, and RTD gates.

Compatibility

• Let/DAG payloads now require a server that reads gfql_query; older servers reject rather than silently return the original graph.

Follow-up: Ubuntu 26.04 support and temporary Ubuntu 24.04 pin removal are tracked in #2246.